Conventionally powered submarines comprise generators which are powered by combustion motors (engines), and these generators produce electrical energy which is stored in the batteries. Furthermore these submarines comprise an electrical drive which, with the help of the electrical energy stored in the batteries, permits the submarine to travel independently of external air. Since however the battery capacity is limited, it is necessary again and again to charge the batteries in the meantime, for which the submarine must surface, or must aspirate the external air required for operation of the combustion motor via a snorkel. The submarine may be located much more easily in these traveling conditions than with a submerged journey independent of external air. For this reason, during a mission one attempts to keep these traveling conditions as short as possible or to conduct these in journey sections in which the danger of a discovery is lower on account of external factors, for example a geographic situation.
Furthermore the behavior of the lead accumulators which are usually applied in submarines is also problematic with regard to the planning of the charging cycles of the battery. Two charging stages result on charging these accumulators or batteries. In the first charging stage one attempts, with an as high as possible current, i.e. as a rule with the maximum available charging power, to charge the battery as quickly as possible. With lead batteries however, on reaching a certain voltage dependent on the acid temperature, an undesirable production of H2 gas occurs, which is not permissible in the submarine. This voltage may not be exceeded in order to avoid this gas production. For this reason, the charging current and thus the charging power must be reduced with an increasing capacity level in the second charging stage. With this, the charging current would theoretically tend to zero in an asymptotic manner only after an infinitely long time, wherein theoretically 100% of the battery capacity is achieved. In practice this second charging stage is therefore terminated after a finitely small charging current. This second charging stage leads to an extension of the charging time, and thus of the time during which the submarine must travel surfaced or by snorkel (worsened indiscretion rate IR=indiscrete charging time fraction in relation to the discharging/charging cycle time).
Since the acid density is dependent on the capacity level and the battery voltage is proportional to the acid density, the time of the mentioned second charging stage is approximately the same, independently of the extent of the previous discharging. This means that even with small discharging quantities one always requires an unfavorable, long second charging stage, by which means the time in which the submarine is not completely protected (indiscretion rate IR), is extended even more.
For this reason, the second charging stage may often not be implemented to the end in practical operation, i.e. the charging must be terminated with a charging of the battery which is not complete. By way of this however the behavior of the battery changes with each renewed discharging of the battery. The specific demand is increased so that the maximal capacity may no longer be fully exhausted. Furthermore with the charging which follows, the gas production sets in very much earlier, i.e. at a low capacity level, so that the unfavorable second charging stage is extended even further. This worsens with each incomplete discharging/charging cycle. Furthermore with a battery capacity which is not sufficient one may no longer travel with the submarine at maximal speed, as a rule, when the battery capacity level with long-term discharging has been discharged below about 45%. This condition should never occur for tactical reasons.
On account of the previously outlined problematic charging behavior of the batteries of a submarine, and due to the operation of a conventional submarine dependent on the external air which is to be avoided where possible, the journey planning and advance planning of the required charging and discharging cycles of the battery is extremely complex and difficult.